NOTE: As their name suggests hints are not exactly Persona 4 Arena cheats, and are usually just simple (but very useful) tips on how you can unlock secret level, features, find hidden areas and such. Hopefully these will make playing Persona 4 Arena even more entertaining.

Action! (Bronze): You performed 100 Furious Actions in Arcade Mode or online.
All-Out Attack to the MAX! (Bronze): You finished an All-Out Attack with a Fatal Counter in Arcade Mode or online.
Amazing, Sensei! (Bronze): You've earned your first online victory! You've bear-ly scratched the surface.
BAM! (Bronze): You used every character's Instant Kill move in Arcade Mode or online!
Belt Collector (Gold): You completed Arcade Mode with all characters.
Black Belt (Silver): You have reached Grade C in Ranked Matches.
C'est Magnifique! (Gold): You completed Score Attack Mode with all characters.
Calm Down. (Bronze): You suffered a Persona Break in Arcade Mode or online. It happens to everybody...
Champion! (Bronze): You completed Arcade Mode with one character.
Combo Crazy (Gold): You have completed all the Challenges! Funky Student would be proud.
Combo Master (Silver): You have completed 300 Challenges.
Die for Me! (Bronze): You performed 30 Fatal Counters in Arcade Mode or online.
Halfway There (Bronze): You completed Stage 6 in Score Attack.
I am Thou... (Platinum): You have mastered P4A.
I Learned a Few Moves (Bronze): You have completed 150 Challenges.I Mastered that One (Silver): You have completed all of one character's Challenges!
Meat Dimension (Bronze): You defeated an opponent in Arcade Mode or online before he Awakened! Now, that's a real challenge!
Null Physical (Bronze): You performed 50 Instant Blocks in Arcade Mode or online.
One More! (Bronze): You performed 20 One More Bursts in Arcade Mode or online.
Per...so......na...! (Bronze): Have you burned your dread? Or reached out to the truth?
Perfect! (Bronze): You had a perfect victory in Arcade Mode or online.
Persona Breaker (Bronze): You created 10 Persona Breaks in Arcade Mode or online.
Replay Reviewer (Bronze): You watched a Replay in the Theater. Are you as good as you remember?
School's Out (Bronze): You've aced all the tests.
Serious Gamer (Silver): You have reached Level 30 by playing online matches!
Speak with Your Fists! (Bronze): You've battled against every character in online matches. You're a grizzly'd warrior now!
Stylish! (Bronze): You performed 100 Auto Combos in Arcade Mode or online.
The Angel (Bronze): Aigis' Story chapter has been told.
The Beast (Bronze): Teddie's Story chapter has been told.
The Boxer (Bronze): Akihiko's Story chapter has been told.
The Captain (Bronze): Yosuke's Story chapter has been told.
The Carnivore (Bronze): Chie's Story chapter has been told.
The Detective (Bronze): Naoto's Story chapter has been told.
The Emperor (Bronze): Kanji's Story chapter has been told.
The Kingpin (Bronze): Yu's Story chapter has been told.
The Queen (Bronze): Mitsuru's Story chapter has been told.
The Unconquerable (Bronze): Yukiko's Story chapter has been told.
They Call Me... (Bronze): You've created a Title for your online matches.
Time for the All-Out Attack! (Bronze): You performed 100 All-Out Attacks in Arcade Mode or online.
Trained in Seclusion (Bronze): You spent 30 minutes in Training in one sitting. Now take it to the streets!
Tres Bien! (Bronze): You completed Score Attack Mode with one character.
Veteran (Bronze): You have completed 100 online matches, win or lose. No bear puns here. Congrats.
Victory Cry (Bronze): You won 30 Rounds in Arcade Mode or Online by using SP Skills or Awakened SP Skills.
Wild (Bronze): You have used all the characters in Arcade Mode or Online. Variety is good for you!
You know the Midnight Channel? (Bronze): Tuned In
You Should Leave Him Be... (Bronze): You were afflicted with every ailment in Arcade Mode or online. Not even Mr. Edogawa can help you.

New Character Story Modes
You initially have 4 characters available for Story mode but if you complete the Story mode with a character and you will unlock story mode for more characters.